1. Field of the Invention
The present invention relates to a level-shift circuit that carries out a level-shift needed for a switch in a half-bridge configuration.
2. Description of the Related Art
As a conventional level-shift circuit, for example, there has been known a level-shift circuit disclosed in a patent document 1 (Published Japanese Translation of PCT International Application No. 2005-512444).
In this level-shift circuit, for a high-side ground potential at a middle point between a high-side switch and a low-side switch in a half-bridge configuration, a voltage level greatly varies within a certain range (e.g., a range of 0 V to 400 V with respect to the ground potential), in synchronization with on/off action of the high-side switch and the low-side switch. Thus, a high-side driver for applying a potential higher than the high-side ground potential is employed to a gate of the high-side switch.
Also, in this level-shift circuit, there are provided a first capacitor and a second capacitor as two passive devices. A rising edge of pulse signal is input into the first capacitor via a first driver, and a rising edge of pulse signal inverted into a falling edge of pulse signal by an inverter is input into the second capacitor. The first capacitor serves to render an electric current to flow by the rising edge of pulse signal. The second capacitor servers to render an electric current to flow by the falling edge of pulse signal.
Namely, each of the first capacitor and the second capacitor temporarily generates an electric current, and sets or resets a latch at a proper time required to properly drive a half-bridge driver or a circuit similar to the half-bridge driver using the temporal electric current. The high-side driver turns on or off the high-side switch using a set signal or a reset signal from the latch.
In the above-described configuration, when a charging voltage of each of the first capacitor and the second capacitor varies according to not a set signal or a reset signal, but instead an external factor, an electric current temporarily flows into or from each of the first capacitor and the second capacitor.
In the above-described configuration, there is a possibility that an inductance load such as a transformer or a reactor is connected to the middle point between the high-side switch and the low-side switch in the half-bridge configuration. Here, it is noted that an inductance component of the inductance load oscillates by voltage fluctuation or current fluctuation due to the switching action of the high-side switch and the low-side switch.
A whole high-side potential of the high-side driver oscillates by this oscillation because a potential at the middle point between the high-side switch and the low-side switch is the high-side ground potential. If the charging voltage of each of the first capacitor and the second capacitor varies according to the external factor, each of the first capacitor and the second capacitor temporarily generates an electric current independent of a set signal or a reset signal. This brings the latch circuit to malfunction, which causes unreliable signal transmission to the high-side switch.